Spectral editing: A quantitative application of spin-echo nuclear magnetic resonance spectroscopy to the study of 27Al in zeolite catalysts

• Published in: Zeolites 1994-02, Vol.14 (2), p.89-100
• Main Authors: Schmitt, Kirk D, Haase, Jürgen, Oldfield, Eric
• Format: Article
• Language: English
• Subjects: alumina bound catalysts; aluminum n.m.r; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; kaolinite theory; Magnetic resonances and relaxations in condensed matter, mössbauer effect; Nuclear magnetic resonance and relaxation; Physics; Spin echoes; Spin-echo editing; steam cracking
• ISSN: 0144-2449; 1873-3115
• DOI: 10.1016/0144-2449(94)90002-7

27Al spin-echo nuclear magnetic resonance (n.m.r.) is used to measure the spin—spin relaxation times, T 2H, for a substantial number of model compounds, and a theory (with no adjustable parameters) based on AIAI dipolar interactions combined with crystallographically determined AIAI distances is used to estimate T 2H. The homonuclear magnetic dipole interaction explains the experimental data reasonably well for compounds with high Al levels, but much less well for compounds with low levels of AI, where structure-specific interactions are important. Such structure-specific interactions are exploited to edit zeolitic AI from the background binder in alumina-bound ZSM-5 and in dealuminated zeolite-Y catalysts containing nonframework (NFW) AI. Editing allows quantitative analysis of the zeolitic components. For dealuminated zeolite Y, it is concluded that peaks assigned by others to “pentacoordinate” AI may actually arise from NFW aluminum, based on the fact that their T 2H is short relative to framework (FW) AI. Theory and experimental results for the technically more demanding measurement of T 2H under conditions of “magic-angle” sample spinning ( MAS) with synchronous sampling are also reported. Spin-echo editing of synchronously sampled 27AI MAS n.m.r. spectra are shown to be useful for determining the FW zeolitic AI content of realistically formulated (kaolinite bound) and steamed/calcined fluidized bed cracking (FCC) catalysts. The loss of framework AI in two series of steamed FCC catalysts is less precipitous than the loss in catalytic activity, as measured by the hexane cracking α parameter.